journal of science effects of single pass and multipass

*Correspondingauthor, [email protected]

GU J Sci 30(4): 514-529 (2017)

Gazi University

Journal of Science

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Effects of Single Pass and Multipass Welding on Austenitic Stainless Steel

Corrosion in Aggressive Environments Lawrence O. OSOBA1, Joshua O. OKENIYI2,*, Benjamin I. POGOSON1, Omoniyi A. FASUBA3 1Department of Metallurgical and Materials Engineering, University of Lagos, Nigeria 2 Mechanical Engineering Department, Covenant University, Ota, Ogun State, Nigeria 3Materials Science and Engineering Department, African University of Science and Technology, Abuja, Nigeria

ArticleInfo

Abstract

This study investigates single pass and multipass (three-pass) welding effects on 304 austenitic stainless steel (ASS) corrosion in acidic/saline environments, to enable a better understanding of the influence of heat affected zone (HAZ) on ASS corrosion susceptibility in the selected commonly used aggressive environments. Tungsten Inert Gas (TIG) welding process was used to weld the samples using 308 stainless steel filler. The corrosion behaviour of the welded samples was evaluated using immersion and potentiodynamic polarization tests. Immersion test evaluation and the analysed results showed that the single pass welds exhibited lower corrosion susceptibility in the acidic environments (H2SO4, HNO3) than in the multipass welded ASS. In contrast, using potentiodynamic polarization evaluation, the multipass welded sample, exhibited lower corrosion susceptibility than the single pass welded ASS in HNO3 environment. Essentially, the same trend of corrosion susceptibility was recorded in NaCl environments irrespective of the evaluation method.

Received:15/04/2016 Accepted: 31/10/2017

Keywords

Austenitic stainless steel corrosion

Single and multipass welding

Corrosion test-data analyses

Acidic and saline environments

1. INTRODUCTION Welding is a well-known traditional method of fabrication and repair for many metallic alloys [1-2]. These metallic alloys include austenitic stainless steel (ASS), a material that finds usefulness in many industrial applications due to its elemental composition-induced desirable properties such as environmental compatibility, excellent corrosion resistance and relatively low cost [3-5]. Tungsten inert gas (TIG) welding is commonly used in industry for sheet joining purposes because it allows greater control over the weldment resulting in stronger and higher quality weld [6]. In addition, the process is quick, economical, clean and free of slag and sputter [7]. Generally, in a weldment or fusion joint, three distinct regions can be identified namely the fusion zone (FZ), the heat affected zone (HAZ) and the base metal [8]. The microstructure developed in the fusion zone depends on the solidification behaviour of the weld pool. Solidification occurs by a process of nucleation and growth depending on the cooling rate. As such, the FZ can be considered as a cast zone because important parameters that determine microstructures in casting, such as growth rate, temperature gradient, undercooling, and alloy composition influences microstructural development in the FZ [9]. Multipass welding in which one layer of the weld is allowed to cool before another weld is performed on top of it is commonly utilized in the industry during fabrication/joining process of thick sections, wear repair, surface hard facing or build-up. During multipass welding, the region adjacent to each new weld deposit experiences short-term high temperature (about 1300 °C) reheats [10], which cause re-melting of the previous beads, and then solidify, generating multiple heat affected zone. The heat affected zone (HAZ) in multipass weld microstructure does significantly vary from that obtained during single pass welding process because of the multiple reheats [11-12].

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515Lawrence O. OSOBA, Joshua O. OKENIYI, Benjamin I. POGOSON, Omoniyi A. FASUBA / GU J Sci, 30(4): 514-529 (2017)

Normally, the microstructure of ASS is mainly composed of austenite under equilibrium condition [13]. However, during welding, a non-equilibrium rapid solidification process due to the high cooling rate often gives rise to an incomplete transformation of austenite resulting in a small amount of delta ferrite that remain in the weld microstructure at room temperature [14-15]. The amount of retained delta ferrite is predominantly dependent on the composition of the filler metal, dilution from the parent metal and the chemical reactions that take place between the weld metal and the slag or surrounding gases. Depending on the ASS composition, ferrite, austenite, martensite or combinations of these can form due to the role of nickel and chromium in promoting the stabilization of austenite and ferrite, respectively. In addition, microstructural inhomogeneity and compositional changes occur in the HAZ adjacent to the FZ during welding. These often results in sensitization that takes the form of segregation of chromium to form M23C6 carbide and other inter-metallic (sigma, Cr30Mo6Ni5; Chi, Cr27Mo12Ni5 and Cr20Mo39Ni3) phases along the grain boundaries. Sensitization, which is a major problem during high temperature processing like welding makes ASS highly susceptible to intergranular corrosion [16-18]. Although several works exist in the literature on corrosion susceptibility of ASS and the problem of its associated sensitization during welding [5,19], however, there are little and/or few information on the effects of multiple heat affected zones generated during multipass welding process on corrosion susceptibility of ASS. Also, more experimental studies are required for a better understanding of the conditions in which multipass welded stainless steel could exhibit lower corrosion susceptibility, or otherwise, than the single pass welded stainless steel. Therefore, the objective of this study was to investigate the effects of single pass and multipass welding on corrosion susceptibility of ASS in commonly used aggressive (i.e. acidic and saline) environments. 2. EXPERIMENTAL The chemical composition of the austenitic stainless steel AISI 304 grade base plate and the 308 stainless steel filler metal used for this research is presented in Table 1. Table 1.Chemical composition(wt%) of the base metal and filler metal

Type Cr Ni Co Mn P S Mo Cu Si Fe C AISI 304 18.72 7.84 0.14 1.78 0.04 0.01 0.27 0.32 0.32 70.3 0.04

(ER 308 SS) 18.15 10.01 – 1.57 0.04 0.03 – – 1.0 69.13 0.07 The base plates were cut into sizes of 120 mm × 20 mm × 8 mm and clean with acetone to remove lubricant and surface contamination. Welding was performed using Lincoln TIG welding machine. Before welding, the base plate were machined to prepare butt joint edges chamfered at 30° and tacked at both ends of the plate to make a single vee groove of 60° while maintaining a consistent root gap of 1.5 mm. The single pass and multipass welding conditions were carried out using a 2.5 mm diameter filler electrode with a current of 100 A and arc voltage of 12 V. During and after the welding, joints were visually inspected for any geometrical non-conformity and weld defects. Samples from the multipass and single pass welded material were cut and prepared for metallographic observation by grinding to 1200 mesh (from 120, 240, 320, 400, 600, 800 Mesh) on silicon carbide paper and finally polished on emery cloth using a suspension of alumina powder to obtain a mirror-like surface. Optical microscopy observation of the welded samples was obtained using CETI metallurgical microscope (Model No. 0703552) after the samples were etched by swabbing in a mixed solution of HCl and HNO3 (aqua regia) in ratio 3:1 for 45 – 60 seconds. The hardness profile of the single pass and multipass welded samples were obtained using Buehler® microhardness tester. Vickers pyramid indenter was used for microhardness measurements along the longitudinal centreline of the weld through the heat affected zone and into the base metal with a testing load of 490 mN and a dwelling time of 10 seconds. For the immersion test, samples of sizes 40 mm × 20 mm × 8 mm were sectioned and prepared according to standard procedure prescribed in ASTM G1 [20]. The coupons were ground from 120, 240, 400, and


600 mesh silicon carbide papers and cleaned with acetone to remove grease and oxidation films, eventually rinsed in distilled water, dried and weighed. Standard grade chemicals and distilled water were used to prepare different concentrations of Sodium Chloride (NaCl) and 1 mol/dm3 (1 M) Sulphuric Acid (H2SO4) and Nitric Acid (HNO3) concentrations. Each of the samples was immersed in marked jars containing 200ml of corrosive test-solution for 840 hr to determine the weight loss at interval of 120 hr. The samples were removed and cleaned in distilled water and ethanol before weighing with a digital weighing machine (Uni Bloc, Type UW1020H and Model No. D482800085) that was sensitive up to 0.001 g. Each test was repeated three times to ensure the reproducibility of the test-results. The test was carried out at room temperature with no agitation and aeration. For potentiodynamic polarization test, samples with the size of 10 mm × 10 mm × 5 mm were mounted in resin and connected with a flexible wire. The mounted samples were then ground, polished, cleaned and rinsed properly. The potentiodynamic polarization test equipment consisted of potentiostat, a stainless steel specimen as a working electrode, a platinum counter electrode and a reference electrode (saturated calomel electrode, SCE). After immersion in the solution, the polarization tests were carried out in three different concentration of sodium chloride (NaCl) and in 1 M sulphuric acid (H2SO4) and nitric acid (HNO3) concentrations at room temperature. Data were recorded using a computer-based data logging system (Autolab PGSTAT 204N) employing NOVA software for corrosion analysis. 3. RESULTS AND DISCUSSION 3.1. Microstructural Analysis The microstructure of the as received austenitic stainless steel as shown in Figure 1 comprises of the austenite phase and delineated grain boundaries [14].

Figure 1.Microstructure of the as received austenitic stainless steel sample Optical images of the FZs and the HAZs of the welded austenitic stainless steels are shown in Figure 2a and b, for the single pass welding and the multi-pass welding, respectively. From the figure 2, microstructures of transformed zone are observed in both samples in the region close and along the fusion boundary [21]. However, the extent of transformation appears to be higher in the multipass weld bead compared to the single pass weld region.


Figure 2.Optical image of FZ and HAZ (a) single pass welded austenitic stainless steel and (b) multipass

welded austenitic stainless steel In comparison with single pass weld metal and HAZ, the multiple HAZ generated during multipass welding is confined to the prior FZ in the welded sample. The single pass welded material appears to retain its wrought structure in the HAZ with a slight modification because of the limited heat cycle it has experienced during welding. In contrast, the multipass weld metal is significantly different in morphology due to the reheated nature of the HAZ embedded in the FZ. Furthermore, it could be observed from the optical image of the HAZ, presented in Figure 3 that the grain size in the HAZ of the single pass weld is coarse, see Figure 3a, while the grain structure in the HAZ of the multipass weld appears more refined, Figure 3b, than that from the single pass weld similar to what has been previously reported [22]. This observation may be attributed to the extent of thermal cycling and consequential recrystallization that occur during multipass welding.

Figure 3.Optical image of HAZ in (a) single pass welded sample (b) multipass welded sample Close observation of the fusion zone microstructure reveals a significant difference in the morphology of the dendritic structure in that in the single pass weld, the dendritic structure are less pronounced compared to the dense formation that occurred in the multipass weld fusion zone because of the increased number of weld passes, and it is in agreement with a previous investigation [23]. As shown in the optical image of the FZ presented in Figure 4a, for the single pass and in Figure 4b for the multipass welded sample, the fusion zone is typically dendritic with lathy ferrite morphology, similar to what has been previously reported [24].


Figure 4.Optical image of the fusion zone showing delta ferrite in (a) single pass (b) multipass welded

sample The variation of the delta ferrite morphology in the FZ of the multipass welded sample is presented in Figure 5, which shows the optical images of the surface (Figure 5a), middle (Figure 5b) and root (Figure 5c) of the multipass welded ASS sample. Close to the surface of the FZ are more of skeletal delta ferrite as shown in Figure 5a while in the middle and close to the root of the fusion zone are more of lathy delta ferrite as shown in Figure 5b and Figure 5c, respectively. Most regions within the fusion zone have a mixed ferrite morphology [23,25-26].


Figure 5.Optical images of the fusion zone in multipass welded samples: (a) face (b) middle (c) root According to previous study [27], the significant effect of welding process on the microstructure that forms in the single and multipass welded material, as discussed above, is expected to have a strong influence on the corrosion susceptibility of weld metal in any aggressive environment. This is because of the significant differences in the intensity and morphology of the precipitate phases formed in the HAZs of the single pass and of the multipass weld metal. 3.2. Micro Hardness Analysis During welding, the generation of thermally induced stresses (heating, cooling and residual) is inevitable owing to the expansion and contraction that occurred during the process [21,28-29]. One of the factors that could possibly alter the susceptibility of welded structure to corrosion is the magnitude of the induced stresses that emanated during welding, which can be related to the microhardness that developed across the weld joint. Therefore, the results of the microhardness evaluation of HAZ in the single pass and the multiple HAZ in the multipass welded materials for this study are presented in Figure 6.


Figure 6.Microhardness of the HAZ in single pass and multiple heat affected zone in multipass welded samples

The results show that the hardness in the heat affected zone of the multipass welded material is higher than that in the HAZ of the single pass welded material [23,28]. This result further supports the formation of retransformed nature of the microstructure formed in the multipass weld metal compared to the essentially coarser microstructure in the single pass welded material. Based on the microhardness results, its influence on corrosion susceptibility of the multipass welded sample and the coarse microstructure observed in the single pass weld was investigated in acidic (H2SO4, HNO3) and basic (NaCl) environment. 3.3. Corrosion Behaviour 3.3.1. Immersion tests and analyses The results of corrosion susceptibility evaluation are presented in Figure 7 for the single pass and the multipass welded ASS materials immersed in different test-environments including 1 M HNO3 (Figure 7a), 1 M H2SO4 (Figure 7b) and 1 M NaCl (Figure 7c) test-solution. From the figure, it is observed that corrosion rate decreases with exposure time in all the test-environments. This can be attributed to the formation of an impervious thin layer of oxide on the surface of the ASS in each corrosive medium that slows down or prevents further attack by the corrosive media [30-32].

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Figure 7.Corrosion effect of single pass and multipass welded ASS immersed in test-solutions of (a) 1 M NNO3, (b) 1 M H2SO4 and (c) 1M NaCl

Numerical detailing of the corrosion test-results through correlation fitting analyses of corrosion rate, CR, as dependent variable [33] versus exposure time, t. Independent variable shows that the corrosion test-responses from the 1 M HNO3 test-system followed the relationships:

( )3

3single pass (HNO ) 10 6354.46 1 4.01CR t−= + (1)

( )3

3multipass (HNO ) 10 7453.84 1 3.87CR t−= + (2)

For these correlation fitting models, correlation coefficient, R, and the Nash-Sutcliffe efficiency, NSE, [34] are respectively 99.10% and 98.21% for the single pass (Equation (1)) or 99.27% and 98.55% for the multipass (Equation (2)) welded ASS samples in the 1 M HNO3 test-medium. These correlation parameters interpret to excellent fitting model efficiency by both of the correlation fitting relationships [35-36]. From the correlation fitting Equation (1) and Equation (2), it could be deduced that the multipass welded ASS metal exhibited slightly higher rate of corrosion susceptibility than the single pass welded ASS, especially at low exposure time. However, this disparity in corrosion susceptibility is so small that increase in exposure time could quickly culminate in similarity of corrosion test-responses by both the single pass and the multipass welded ASS material, which is what is observable from the test-results plotted in Figure 7a. In similar manner, the corrosion test-responses from the 1 M H2SO4 test-system followed the relationships:

0.000.020.040.060.080.100.120.140.16

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( )2 4

3single pass (H SO ) 10 12,821.36 1 6.11CR t−= + (3)

( )2 4

3multipass (H SO ) 10 17,253.96 1 3.06CR t−= + (4)

The respective parameters (R, NSE) for these fitting models are (99.56%, 99.12%) for the single pass (Equation(3)) or (99.89%, 99.78%) for the multipass (Equation(4)) welded ASS material immersed in the 1 M H2SO4 test-solution. Also, these parameters interpret to excellent model efficiency by the correlation fitting models of Equation (3) and Equation (4). The correlation fitting models also showed that the multipass welded ASS exhibited higher rate of corrosion susceptibility in the H2SO4 medium, at given exposure time, which is even of higher order than was observed with the HNO3 test-system. This contrast followed from the well-known tendency of nitric acid (HNO3) at forming passive film on the surfaces of materials exposed to it, although it also known that should such passive film be broken, abrupt change in corrosion rate could ensue. For the H2SO4 solution, the multipass welded ASS attains high corrosion rate faster than the single pass welded ASS, although, the disparity in corrosion rate still exhibit tendency of disappearing with increasing exposure time, as could also be observed in Figure 7b. The corrosion test-responses from the 1 M NaCl test-solution assumed the relationships:

( )3single pass (NaCl) 10 21,694.58 1 2.63CR t−= − (5)

( )3multipass (NaCl) 10 12,494.17 1 2.25CR t−= − (6)

These exhibited correlation parameters (R, NSE) that respectively evaluate to (99.87%, 99.74%) for the single pass (Equation(5)) or (99.43%, 98.87%) for the multipass (Equation(6)) welded metal immersed in the 1 M NaCl test-solution. These are parameter values that also indicate excellent fitting model efficiency by the correlation equations. In contrast to the previous results, however, the correlation fitting models showed that the multipass welded ASS exhibited much lesser rate of corrosion susceptibility at given exposure time than the single pass welded ASS material in the 1 M NaCl test-environment. The single pass welded ASS corrodes faster than the multipass welded ASS in the 1 M NaCl. This is well corroborated by the experimental corrosion test results plotted in Figure 7c. Based on the foregoing results and consequent interpretations, it is considered appropriate to study analyses of variance (ANOVA). This is to enable the testing significance of the obtained correlated relationships between the dependent variable, CR, and independent variable, t. Based on the results from previous ANOVA studies [33,37] and presented for the test-systems of 1 M HNO3 in Table 2, for 1 M H2SO4 in Table 3 and of 1 M NaCl in Table 4. The results in the tables showed that p-value is << 0.05 for each of the test-systems, an indication that a significant relationships exist between the dependent variable CR and t, for each of the test systems of 1 M HNO3, 1 M H2SO4, and 1 M NaCl. Table 2.Analyses of variance for the correlation fittingfrom the 1 M HNO3 test-system ASS Welded Model

Source of Variation df SS MS F p-value

Single Pass Treatment 1 1.5461 × 10–3 1.5461 × 10–3 274.4541 1.46 × 10–5 Residual 5 2.8167 × 10–5 5.6335 × 10–6 Total 6 1.5743 × 10–3

Multipass Treatment 1 2.1274 × 10–3 2.1274 × 10–3 339.2703 8.68 × 10–6 Residual 5 3.1353 × 10–5 6.2705 × 10–6 Total 6 2.1588 × 10–3


Table 3.Analyses of variance for the correlation fitting from the 1 M H2SO4 test-system ASS Welded Model




Table 4. Analyses of variance for the correlation fitting from the 1 M NaCl test-system ASS Welded Model




Figure 8 shows the results of average corrosion rate evaluations for the single pass and the multipass welded ASS material in the different test-immersion environments of 1 M HNO3, 1 M H2SO4 and 1 M NaCl. This figure vividly highlights the slight disparity in the corrosion susceptibility in HNO3, the higher disparity in corrosion susceptibility in H2SO4, and the contrasting change in corrosion susceptibility behaviour in NaCl by the single pass and the multipass welded ASS metal.

Figure 8.Experimental and predicted average corrosion rate of single pass and multipass welded

austenitic stainless steel in different immersion environments The observed tendency of reduced corrosion susceptibility in the NaCl test-environment is investigated in this study by varying the concentrations of NaCl test-solution (using 0.5 M NaCl and the 0.75 M) for immersion test of the single pass and the multipass welded ASS material, in addition to the 1 M NaCl.

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The results of corrosion susceptibility evaluation in these additional concentrations of NaCl test-environments is presented in Figure 9, i.e. Figure 9a and 9b for the 0.5 M NaCl and 0.75 M NaCl test-systems, respectively. From figure 9, it is noted that the trend of decreasing corrosion rate with increasing exposure time, as previously reported above from all the corrosion test-environments, also predominates. Furthermore, in agreement with observed results from the 1 M NaCl test-environment as seen in Figure 7c, Figure 9 showed that the corrosion rate for the single pass welded ASS are consistently higher than that of the multipass welded ASS in both the 0.5 M NaCl and the 0.75 M NaCl test-environment. The results confirm that the multipass welding procedure do not degrade the corrosion resistant property of the ASS material in the NaCl environment, but rather exhibited an apparent improvement in the corrosion resistant property compared to the single pass welded metal in the same environment.

Figure 9.Corrosion effect of multipass and single pass welded ASS in (a) 0.5M NaCl (b) 0.75M NaCl The lower corrosion rate, CR, by the multipass welded ASS than that from the single pass material, for given NaCl concentration, ρ, and immersion time, t, is further corroborated by the correlation fitting Equation (7) for the single pass and Equation (8) for the multipass model. For these fittings, the corrosion rate, taken as dependent variable [33], was correlated with functions of the concentration, ρ, of the NaCl test-solution (including the 1 M NaCl system) and of the immersion time, t, taken as the independent variables, for obtaining the relationships:

( )3 27.49 146.96single pass 10 41.41 0.2 ln 809.27CR t tρ ρ− − − = + + ⋅ + (7)

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( )3 5.09 87.4multipass 10 1.62 0.13 ln 498.71CR t tρ ρ− − = + + ⋅ + (8)

From these correlation fitting equations, it is deduced that, for given NaCl concentration, higher value of corrosion rate will be initiated faster in the single pass welded material than in the multipass welded material. These suggest a faster corrosion failure tendency by the single pass welded ASS than the multipass welded ASS. Furthermore, it is worth noting that the correlation coefficient, R, and the Nash-Sutcliffe efficiency, NSE, for the correlation fittings in Equation (7) and Equation (8), are respectively 99.26% and 98.53% for the single pass model, 98.75% and 97.52% for the multipass model. These fitting parameters also indicate an excellent model efficiency by each of the correlation models based on previous work [35-36]. The analyses of variance for the correlation fittings, shown in Table 5, indicate that ANOVA p-values << 0.05 for both of the welded models, which implies that it could not be rejected that there is significant relationship between the dependent and independent variables, for the models. Table 5. Analyses of variance for the correlation fitting from the different NaCl test-systems ASS Welded Model


Single Pass Treatment 4 0.0497 0.0124 268.2739 1.93 × 10–14 Residual 16 7.4119 × 10–4 4.6324 × 10–5 Total 20 0.0505

Multipass Treatment 4 0.0158 3.9516 × 10–3 157.1677 1.27 × 10–12 Residual 16 4.0228 × 10–4 2.5142 × 10–5 Total 20 0.0162

The average corrosion rate of the single pass and of the multipass welded ASS by the experimental model and the predicted model from the correlation fitting are presented in Figure 10, for the different concentrations of NaCl test-solutions employed in the study. This also includes the average corrosion rate from the 1 M NaCl test-system, for fostering comparisons across the different concentrations of NaCl employed for the study. By these, it could deduced from the Figure 10 that the predicted model exhibited agreements with the experimental model and both of these models further confirmed that the multipass ASS welded materials exhibited lower corrosion susceptibility than the single pass ASS welded materials in the NaCl environment.

Figure 10.Experimental and predicted average corrosion rate of single pass and multipass welded

austenitic stainless steel in varying concentration of NaCl

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3.3.2. Corrosion rate by potentiodynamic polarization test In order to verify and clarify the observed results from immersion test, potentiodynamic polarization measurements was carried out, because it is a fast, simple and reliable ways of selecting/evaluating corrosion susceptibility of materials in specific environment. The plots from these potentiodynamic test-experiments are presented in Figure 11, for the 1 M HNO3, the 1 M H2SO4, the 1 M NaCl and the 0.5 M NaCl corrosive test-systems. In addition, results of the corrosion current density (icorr), corrosion potential (Ecorr) and polarization resistance for each of the tested ASS samples were determined for each medium from the potentiodynamic polarization curves (using Tafel extrapolation method). These results are summarized in Table 6.

Figure 11.Potentiodynamic polarization curves of single pass and multipass welded austenitic stainless

steel in (a) 1 M HNO3 (b) 1 M H2SO4 (c) 1 M NaCl (d) 0.5 M NaCl solution Table 6. Electrochemical corrosion parameters obtained from potentiodynamic polarization curves for various samples in varying concentration and media

Corrosive test-system

icorr (µA/cm2) Ecorr (mV) Polarization Resistance (Ω)

Single Pass

Multipass Single Pass Multipass Single Pass Multipass

1M HNO3 18.80 16.80 –32.11 –86.01 1712.40 1019.20 1M H2SO4 27.63 69.58 –252.99 –267.85 512.82 506.62 1M NaCl 0.502 0.132 –708.91 –672.21 3547.60 1243.10 0.5M NaCl 2.37 1.15 –334.92 –353.67 11889.00 19420.00 The potentiodynamic polarization test-results followed the trend observed in the immersion corrosion tests evaluation, with an exception of corrosion susceptibility in the HNO3 test- environment. In contrast

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to the results obtained from the immersion test, the multipass welded ASS exhibited lower corrosion susceptibility than the single pass welded sample, as observed in the value of the corrosion current density, 16.80 µA/cm2 which is rather lower than that observed in single pass 18.80 µA/cm2 although the polarization resistance is comparably higher in multipass weld. . This observation may be related to the inequality in the degree of passivation experience by multipass and the single pass welded ASS in HNO3 environment during potentiodynamic tests. The corrosion potential confirms this suspicion of passivation disparity, especially, from the consideration that although the corrosion current density of the multipass welded ASS was lower, its corrosion potential was more negative than what was obtained from the single pass welded ASS. From the remaining tests, however, the following agrees with the results from the immersion test-method ensued:

i. The multipass welded ASS exhibited higher corrosion susceptibility than the single pass welded ASS in the 1 M H2SO4 test-environment based on all parameters of corrosion current density, polarization resistance and corrosion potential;

ii. The multipass welded ASS exhibited lower corrosion susceptibility than the single pass welded ASS in the 1 M NaCl test-environment based on corrosion current density and the corrosion potential considerations;

iii. The multipass welded ASS exhibited lower corrosion susceptibility than the single pass welded ASS in the 0.5 M NaCl test-environment based on all parameters of corrosion current density, the polarization resistance and corrosion potential.

The influence of multiple heat affected zone occurrence due to multipass welding using immersion and potentiodynamic polarization test shows that the practice of multipass welding does not negatively aggravate corrosion sensitization of austenitic stainless steel weld metal in NaCl and in HNO3 environments. This observation may be attributed to the role of elemental chromium segregation on corrosion susceptibility in wrought ASS where the microstructure in the HAZ of single pass welded material is influenced by the formation of M23C6 along grain boundaries [38-39]. The as-cast delta ferrite precipitate phase in the multiple HAZ embedded in the FZ of the multipass weld appears to have considerable influence its susceptibility to corrosion. In essence, in the single pass welding process, the wrought microstructure of the HAZ where M23C6 precipitates are formed owing to chromium segregation and are known to cause sensitization with associated degradation of corrosion resistance. Furthermore, as observed in the current study, the multipass welded material have a significant delta ferrite precipitation in the multiple HAZ embedded in FZ compared to the single pass weld metal. In addition, the multiple heat affected zone can possibly lead to preheating and homogenization of the previous weld pass in the multipass welded samples and this may also have contributed to the improvement in the corrosion resistance and passivity with time in the multipass weld metal. 4. CONCLUSİON In this study, single pass and multipass welded austenitic stainless steel samples were used to investigate the influence of heat affected zone produced during the welding on the corrosion susceptibility of austenitic stainless steel. From the results in the study, it can be concluded that:

• The multipass weld metals have a significant delta ferrite morphology precipitation compared to the single pass welded material having a wrought alloy microstructure;

• The multipass welded material has a comparatively higher hardness value in the fusion zone and heat affected zone compared to the single pass welded material;

• Analysed corrosion test-results detailed corrosion susceptibility and exhibited results that agreed with experimental results;

• Experimental and analysed corrosion test-results showed that although the single pass welded ASS exhibited lower corrosion susceptibility in H2SO4 than the multipass welded ASS, however, the multipass welded ASS exhibited lower corrosion susceptibility in NaCl environments, as well as in the HNO3 test-environment using the evaluation.

• These results indicated that the practice of multipass welding does not necessarily aggravate corrosion susceptibility of austenitic stainless steel material in HNO3 and in NaCl.


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